Exhaust gas recirculation (EGR) has been used in recent years to reduce NOx emissions in light duty diesel engines. EGR involves diverting a fraction of the exhaust gas into the intake manifold where the recirculated exhaust gas mixes with the incoming air before being inducted into the combustion chamber. EGR reduces NOx because it dilutes the intake charge and lowers the combustion temperature. A practical problem in fully exploiting EGR is that, at very high levels, EGR suppresses flame speed sufficiently that combustion becomes incomplete and unacceptable levels of particulate matter (PM) and hydrocarbons (HC) are released in the exhaust. This transition to incomplete combustion is characteristically very abrupt due to the highly nonlinear effect of EGR on flame speed. In a transient operating environment, it is particularly difficult to reliably approach this instability limit without occasionally producing undesirable bursts of HC and PM emissions. The result is that diesel engines must be typically operated significantly below their maximum EGR potential, thus penalizing NOx performance. The effect of EGR on development of combustion instability and particulate formation was discovered and options for maximizing the practical EGR limit were identified. The dynamic details of the combustion transition with EGR and how the transition might be altered by appropriate high-speed adjustments to the engine is taught herein. Using the combustion diagnostic of this invention, one can alter the effective EGR limit (and thus NOx performance) by using advanced engine control strategies. All experiments described here were performed on a modern turbo-charged, direct-injection automotive diesel engine. This engine was selected on the basis that it is likely to reflect the EGR response of more advanced diesel engines proposed for automotive use. The results of this study are applicable to stationary CIDI engines, especially those experiencing transient load and/or speed demands.
In general, a direct-injection diesel engine injects a fuel into a combustion chamber with high pressure and high temperature at around the top dead center of the compression stroke of cylinder, so that the fuel is burned by its self-ignition. Here, the fuel injected into the combustion chamber proceeds being divided (atomized) into minute liquid drops by a collision with air having high density, and forms substantially a cone-shape fuel spray. The fuel evaporates from surfaces of the fuel drops and forms a fuel mixture by involving air surrounding mainly a front end and a periphery of the fuel spray. Then, the fuel mixture is self-ignited when it becomes a certain condition with its appropriate concentration and temperature necessary for an ignition, and begins to burn (premixed combustion). Then, it is considered that the portion beginning to burn becomes a core and diffusion combustion is performed involving surrounding fuel vapor and air.
In such a normal combustion of the diesel engine (hereinafter, referred to as diesel combustion), the initial premixed combustion may be followed by the diffusion combustion that burns most part of fuel. Here, nitrogen oxides (NOx) is produced at a portion in which an air excessive ratio λ is nearly 1 in the fuel spray (fuel mixture) having un-homogeneous concentration due to a rapid generation of heat. Also smoke is produced at a portion in which a fuel concentration is too high due to a lack of air. Conventionally, some measures to reduce NOx or smoke are taken, such as recirculating part of an exhaust gas into an intake air (Exhaust Gas Recirculation, hereinafter, referred to as EGR) and increasing injection pressure of fuel.
Recirculating the inert exhaust gas into the intake air system by EGR may suppress production of NOx by decreasing combustion temperature but, on the other hand, promote production of smoke with a large amount of EGR decreasing oxygen in the intake air. Further, increasing injection pressure of fuel may promote minute fuel spray and improve air utilization rate by increasing penetration of the fuel spray, resulting in suppression of smoke, but, on the other hand, it may make a condition where NOx is produced easily. In other words, the conventional combustion of diesel engine provided a trade-off relationship on NOx reduction and smoke reduction, so that it was difficult to reduce both NOx and smoke coincidentally.
In contrast, new combustion modes have been recently proposed that provide a combustion state consisting of premixed combustion mainly by advancing the timing of fuel injection and thereby reducing NOx and smoke coincidentally and greatly. These are generally known as the name of diesel premixed combustion or premixed compression ignition combustion. This is, for example, a new combustion mode, in which a large amount of exhaust gas is recirculated by EGR and a fuel is injected at relatively early timing of the compression stroke of cylinder to mix with air sufficiently, so that the premixed mixture is self-ignited at the end of the compression stroke of cylinder and burns (for example, as shown in Japanese Patent Laid-Open Publication No. 2000-110669).
It is preferable that the rate of recirculated exhaust gas into intake air by EGR (EGR ratio) at such combustion state is set at a much higher level than that at the above-described diesel combustion. That is, a large amount of exhaust gas having larger thermal capacity than that of air is mixed and thereby density of fuel and oxygen in the premixed mixture is reduced, and as a result, the timing of self-ignition of the premixed mixture may be delayed until near the top dead center of compression of cylinder (TDC) by extending its delay time of ignition. Further, the inert exhaust gas disperses evenly around fuel and oxygen in the premixed mixture and absorbs the heat by combustion, and thereby the production of NOx may be suppressed greatly.
However, because increasing the rate of recirculated exhaust gas in the intake air by EGR means decreasing the amount of air in return, it may be difficult to perform the above-described combustion at an engine operating area where the engine load is relatively high. Thus, conventionally, when the engine operation is at relatively low load, an early fuel injection like the above is performed and EGR ratio is controlled higher than a first predetermined value that is relatively high, resulting in premixed compression ignition combustion. Whereas, when the engine operation is at relatively high load, fuel is injected at around the top dead center by changing fuel injection mode, resulting in diesel combustion.
In the meantime, in a case where the engine combustion mode is changeable between the premixed compression ignition combustion and the diesel combustion, problems exist such as a transient deterioration of exhaust gas condition at its changing and an occurrence of large noise. That is, when changed from the premixed compression ignition combustion to the diesel combustion, the EGR ratio is changed from a state where it is higher than the first predetermined value to another state where it is lower than the second predetermined value, by reducing the amount of recirculated exhaust gas by EGR. Here, if only fuel injection mode is changed at once to its injection at around TDC for the diesel combustion, the combustion consisting of the diffusion combustion mainly is performed along with an excessive EGR ratio because controlling the amount of exhaust gas recirculation needs a certain time. As a result, smoke is produced.
On the other hand, in changing from the diesel combustion to the premixed compression ignition combustion, if only fuel injection mode is changed to its early injection when the EGR ratio is not sufficiently high, the fuel may be self-ignited at the too-early timing because adjusting the amount of exhaust gas recirculation needs a certain time as well. As a result, considerably large noise of combustion is produced and an increase of NOx is produced rapidly as well. In addition, a large amount of smoke is produced by combustion of fuel having an insufficient mixture with intake air.
In view of the above-described problems, the present invention has been devised to diagnose the combustion condition of diesel by applying an effective algorithm to better sense and control combustion of a diesel engine in which its combustion is changeable between a first combustion state where its combustion consists of premixed combustion mainly (for example, premixed compression ignition combustion described above) and a second combustion state where its combustion consists of diffusion combustion mainly (for example, conventional diesel combustion).